1. Field of the Invention
The present invention relates to an image forming apparatus preferably applied to a monochromatic or color printer having a function of thermally fixing a toner image formed on a recording medium, a facsimile apparatus of the same type, a digital copying machine of the same type, a complex machine of them or the like.
2. Description of the Related Art
A digital copying machine has heretofore been used in many case, when an image is formed based on original image data obtained by reading an original image. In this type of copying machine, a direct-current motor (hereinafter referred to as the DC motor) which drives a sheet conveying system has been used. For example, when the original image is read by a scanner or the like, a sheet conveying system driven by a DC motor such as a stepping motor conveys an original to an original reading unit, or a sheet having a desired size is conveyed to an image forming unit from a sheet supply cassette.
The original image data read by the scanner is subjected to image processing such as γ correction, zooming, space filter, and image compression. Here the original image data subjected to the image processing is transferred to a printer. The printer forms an image on a predetermined sheet based on the original image data. At this time, an electrostatic latent image based on the original image data is formed on a photosensitive body uniformly charged by a charging unit. This electrostatic latent image is developed by a developing unit. A toner image is formed on the photosensitive body by performing the charging, exposing, and developing, and is transferred onto the sheet by a transfer unit. The toner image transferred onto the desired sheet is thermally fixed by a fixing unit. As a result, the original image can be copied.
Additionally, this type of copying machine is provided with a power supply system for supplying power to the DC motor or the fixing unit which drives the sheet conveying system. FIG. 39 is a block diagram showing a constitution example of a power supply system 10 mounted on a conventional digital copying machine or the like.
In FIG. 39, an alternating-current power supply 1 is connected to a current limiter (LMIT) 2, and a current (user current) I supplied from the alternating-current power supply 1 is limited to, for example, 10 A, 15 A or the like, when a digital copying machine is used. The current limiter 2 is connected to a direct-current power supply 3 and a fixing unit 7 through a current detector 4. The direct-current power supply 3 supplies a direct-current power (current Id) to a direct-current load circuit 5 such as a DC motor. When a primary side and a secondary side are defined by this direct-current power supply 3, the primary side is connected to the alternating-current power supply 1, and the secondary side is connected to the direct-current load circuit 5.
The current detector 4 is connected between the primary side of the direct-current power supply 3 and the current limiter 2, and detects the use current (hereinafter referred to also as a primary-side current) I supplied from the alternating-current power supply 1 to output a primary-side current detection signal SP1. The primary-side current I is obtained by adding up a current of the direct-current power supply 3 on the primary side and a current which flows into the fixing unit 7. The current detector 4 is connected to a power control unit 6, the primary-side current detection signal SP1 is input, and the current detector 4 controls power supply of the fixing unit 7 connected to the alternating-current power supply 1 based on this primary-side current detection signal SP1.
Next, an operation example of the power supply system 10 according to a conventional system will be described.
FIG. 40 is a waveform diagram showing a control example of the primary-side current I supplied from the alternating-current power supply. In FIG. 40, the ordinate indicates an amplitude of the primary-side current I, and the abscissa indicates time t. In FIG. 40, a waveform shown by a bold solid line indicates the primary-side current I at a non-control time, and a waveform shown by a bold broken line indicates the primary-side current I after fixing power control. A thin solid line indicates a limit value which limits the use current supplied from the alternating-current power supply 1. The limit value is set to 10 A, 15 A, 20 A or the like (15 A in Japan). A thin broken line indicates a control threshold value set by the current detector 4. In FIG. 40, M indicates a margin for controlling the primary-side current I, and is given by a difference between the limit value and the control threshold value.
The power supply system 10 of the conventional system relates to a case where the primary-side current I supplied from the alternating-current power supply 1 is limited, and the power supply system 10 executes a supply control of a fixing power based on the limit value to control threshold value. For example, the power control unit 6 monitors the primary-side current I through the current detector 4 in such a manner that the primary-side current I supplied from the alternating-current power supply 1 does not exceed the control value. As a result of the monitoring, when the power control unit 6 detects the primary-side current I exceeding the control threshold value, the power control unit 6 outputs, to the fixing unit 7, a power control signal SP2 for setting the primary-side current I to be not more than the limit value. The fixing unit 7 executes fixing/heating based on the power control signal SP2. Accordingly, the power can be supplied to the direct-current power supply 3, the fixing unit 7 and the like connected to the alternating-current power supply 1.
In a rising portion (direct-current load increase time) of the waveform of the primary-side current I shown in FIG. 40, a time when the waveform crosses the control threshold value corresponds to a control start point of supply of fixing power. In a falling portion (direct-current load decrease time) of the waveform, the time when the waveform crosses the control threshold value corresponds to the control start point of the fixing power supply. In general, when the current Id to the direct-current load circuit 5 fluctuates, the influence is reflected in the primary-side current I. It is known that a time when this influence is propagated to the primary side from the secondary side of the direct-current power supply 3 depends on a power capacity, and requires about 10 ms.
It is to be noted that with regard to the above-described power supply system, a power control device is disclosed in Patent Document 1: Japanese Patent Application Laid-Open No. 10-274901 (see page 3 and FIG. 1). This power control device is mounted on an image forming apparatus having a direct-current power supply and a fixing unit, and is constituted in such a manner as to detect a current input into the image forming apparatus. The fixing power is controlled in such a manner that this current is not more than a certain value. When this device is mounted, power consumed by the image forming apparatus can be efficiently distributed, and rising time of power control can be shortened.
Moreover, an image forming apparatus, a method of controlling the apparatus, and a storage medium are disclosed in Patent Document 2: Japanese Patent Application Laid-Open No. 2002-268446 (see page 3 and FIG. 1). This image forming apparatus comprises a current detection unit, a reader (an original reading unit), and a heater (a fixing unit). The apparatus detects a current which flows into the reader from the alternating-current power supply, and executes power control of the heater based on current detection information. When the apparatus is constituted in this manner, the current consumed by the whole image forming apparatus can be suppressed to be not more than a predetermined value.
Furthermore, a fixing heater energization device has been disclosed in Patent Document 3: Japanese Patent Application Laid-Open No. 2003-177629 (see page 2 and FIG. 1). This fixing heater energization device is mounted on the image forming apparatus having a direct-current power supply, and detects a current input into the image forming apparatus. The fixing heater energization device controls the fixing power in such a manner that this current is not more than a certain value. When the device is constituted in this manner, performance of a load circuit or the like connected to the direct-current power supply is sufficiently usable.
Additionally, according to the digital copying machine on which information conventional power supply system is mounted, there are problems as follows.
(i) The power supply system in which the current detector described in Patent Documents 1 to 3 is disposed on the primary side of the direct-current power supply 3 needs to take much margin of the primary-side current I. This is because a current fluctuation in the direct-current load circuit 5, to be reflected in the primary-side current I, requires about several tens of ms, and a power control range for compensating for this delay time is enlarged. Therefore, since much margin of the primary-side current I is taken, much power cannot be supplied to the fixing unit 7 within a limit of the primary-side current I. Accordingly, since the power that can be supplied fully within the limit value of the primary-side current I cannot be used, an amount of power that can be drawn/assigned to the fixing power is reduced.
(ii) The power control unit 6 cannot start the fixing power supply control from when a current fluctuation is generated in the direct-current load circuit 5 until the primary-side current I exceeds the control threshold value. That is, although the current fluctuation is generated in the direct-current load circuit 5, the primary-side current detection signal SP1 cannot be obtained from the current detector 4, and therefore the power control unit 6 waits for the fixing power control. Therefore, large delay is generated from when the primary-side current I exceeds the control threshold value, and the control is started until an effect of the fixing power control appears. By this constitution, a drop width of the fixing power increases from a power supply state immediately before the control at a time when the primary-side current I exceeds the control threshold value.